1. Field of the Invention
The present invention relates to a code division multiplex (CDM) communication system.
2. Description of the Related Art
Bidirectional optical communication can be conducted by linking a network operator and a plurality of subscribers through a passive optical network (PON). In PON terminology, the signal transmitting and receiving apparatus on the network operator's side of the network is called an optical line terminal (OLT), and the signal transmitting and receiving units on the subscribers' side of the network are called optical network units (ONUs).
A PON system has a star network topology centered on a passive optical coupler (star coupler) that couples the single optical transmission path leading to the OLT to the plurality of optical transmission paths leading to the ONUs. The main advantage of a PON system is that by sharing the optical transmission path to the OLT among a plurality of subscribers, it can reduce system equipment costs. General descriptions of PON systems can be found in Ohnishi et al., ‘Isanetto PON shisutem’ (Ethernet PON system), Fujikura Giho, No. 102, April 2002, pp. 18-21, and Ashi et al., ‘PON Based All Fiber-Optic Access System for High-speed Multimedia Services’, Hitachi Review, Vol. 48, No. 4, 1999, pp. 229-233.
When ONUs are referred to in the following description, it will be understood that they are a group of ONUs connected through a single passive optical coupler to a single OLT. Communication between the OLT and N ONUs will also be referred to as 1-to-N communication, where N is an integer greater than one. Communication from the OLT to the ONUs will be referred to as downstream communication, and communication from the ONUs to the OLT will be referred to as upstream communication.
The conventional optical access network systems described by Ohnishi et al. and Ashi et al. employ time division multiplexing (TDM), and identify signals transmitted from individual subscribers by controlling the time slots in which they are transmitted. PON optical access network systems may also employ code division multiplexing (CDM), which offers a variety of advantages. A PON system using CDM will also be referred to below as a CDM-PON system.
In CDM communication, the transmitter encodes a signal to be transmitted, and the receiver decodes the received signal by using the same code as in the transmitter. One of the advantages of CDM transmission is its inherent security, in that all communication is encoded with a key shared by the transmitter and receiver (the code itself is the key). Another advantage of CDM transmission is that the multiplexed signals are all transmitted at the same time, enabling a large amount of data to be transferred while conserving communication resources such as wavelengths and time slots. Tamai et al. describe an exemplary CDM-PON system in Japanese Patent Application Publication No. 2007-228134 and disclose a ranging method by which signal timing is adjusted to compensate for the different distances between the OLT and the individual ONUs.
A CDM-PON communication system requires clock signals for ranging, decoding, gating, and other operations. TDM transmission and receiving systems generally extract clock signals from TDM signals by detecting the modulation frequency of the TDM signal. The clock signal is extracted by a device termed a clock data recovery (CDR) device. CDR devices using a phase locked loop (PLL) are well known.
A TDM signal, however, is a bi-level digital signal, whereas a CDM signal is a multi-level signal with more than two levels. Conventional CDR devices are designed to operate with bi-level signals and cannot readily recover clock signals from CDM signals.
Although CDR generally refers to both clock and data recovery, CDR is sometimes used to refer only to clock recovery, and that usage will be followed herein.